The present invention relates to a process for purifying wet process phosphoric acid, and more particularly, purification of wet process phosphoric acid to a food grade phosphoric acid.
Until recently, all pure phosphoric acid suitable for application in food products was manufactured by the hydration of phosphoric anhydride obtained by the oxidation of elemental phosphorus: EQU P.sub.4 + 50.sub.2 .fwdarw. P.sub.4 0.sub.10 EQU p.sub.4 0.sub.10 + 6h.sub.2 o .fwdarw. 4h.sub.3 po.sub.4
in this process, the phosphoric anhydride formed is hydrated immediately in the reactor systems where phosphorus is burned. The acid obtained, known as "thermal acid", is then treated with hydrogen sulfide or sodium hydrosulfide to remove heavy metal impurities as insoluble heavy metal sulfides. After filtration, the acid would be diluted to 75, 80 or 85% H.sub.3 PO.sub.4 for commercial applications. The drawbacks of the thermal method are the requirement of relatively expensive capital equipment, larger amounts of electrical energy, and pollution control. Because of the wide gap between the price of wet process phosphoric acid and the price of electrothermal acid made from elemental phosphorus, the purification of wet process phosphoric acid has received increasing attention.
The production of "wet process" phosphoric acid by the treatment of phosphate rock with sulfuric acid is well known in the art and is described for example in Waggaman, Phosphoric Acid, Phosphates and Phosphatic Fertilizers, at pages 174-209, Hafner Publishing Company (2nd Edition, 1969). The production of wet process phosphoric acid by the treatment of phosphate rock with acids other than sulfuric acid, such as hydrochloric acid and nitric acid is also well known in the art and is described in Slack, Phosphoric Acid, Volume 1, Part 2, at pages 889-926, Marcel Dekker, Inc. (1968).
Regardless of what acid is used to produce the wet process phosphoric acid, a considerable amount of dissolved and suspended impurities such as silica, aluminum, magnesium, vanadium, fluorine, chlorine, calcium, arsenic, lead, iron and other organic and inorganic inpurities are contained in the acid. The resulting wet process phosphoric acid contaminated by the aforesaid impurities has relatively limited uses, primarily in the fertilizer industry where such impurities are not only not harmful, but can be considered a desirable source of trace elements.
The prior art does contain methods for purifying wet process phosphoric acid, however, these methods are not entirely satisfactory, primarily due to the fact that they do not produce phosphoric acid as pure as that produced by the well known thermal process. In addition, these prior art processes are costly and tedious, and because of certain steps such as centrifugation, distillation, and the like, which require large amounts of energy and costly equipment, these processes are not economical.
It has, therefore, been an objective of the phosphoric acid industry to provide an efficient economical method for purifying wet process phosphoric acid in order to obtain a product that can be used in foods and pharmaceuticals, with purity as a critical requirement.
The Food Chemicals Codex, 2nd Edition (1972), published by the National Academy of Sciences in Washington D.C., has compiled a list of standards for food grade chemicals that have been given official recognition by the Food & Drug Administration. At page 616, requirements for food grade phosphoric acid are set forth as follows:
A colorless, odorless solution of H.sub.3 PO.sub.4, usually available in concentrations ranging from 75% to 85%. It is miscible with water and with alcohol. PA1 Specifications: PA1 assay. Not less than the minimum or within the range of percent claimed by the vendor. PA1 Limits of Impurities PA1 Arsenic (as As). Not more than 3 parts per million (0.0003 percent). PA1 Fluoride. Not more than 10 parts per million (0.001 percent). PA1 Heavy Metals (as Pb). Not more than 10 parts per million (0.001 percent).
The present invention has achieved an efficient and commercially economical method for puifying wet process phosphoric acid to food grade acid. It is characterized by high yields, it is not energy dependent and is simple in operation. In addition, all materials can generally be reused after conditioning.